Several patents relate to chemical treatment of sewers, for killing tree roots and other organic growth, including U.S. Pat. No. 3,741,807; 3,880,176; and 3,892,588. Treatment is sometimes done by injecting foam containing phytotoxic material into the sewer, as is illustrated by U.S. Pat. No. 3,874,926; 4,025,360; 4,083,384; and 4,556,434.
The treating chemical, when not a foam, was usually applied by completely filling a pipe with an aqueous solution or suspension of the chemical and then allowing a period for soaking. Soaking is best for relatively small-diameter sewer pipes; the pressure caused by filling the pipe forces the solution out of the cracks and into the joints and extends the protection well beyond the line; it increases residual control by dichlobenil herbicide which retards root regrowth. However, soaking becomes expensive when the pipes are large, because the sewer pipe must be completely filled with the expensive treating liquids. Storm sewers, which often need the treatment, are often so large that the soaking method is completely impractical and uneconomical.
The greatest drawback to soak treatment, however, is the ever-present danger of flooding adjacent structures. Therefore, this method cannot be used where either hills or basements are present. Because of these limitations other methods were devised which increasingly diminished the possibilities of spills.
Alternatively, the treating material was in the past sprayed onto the pipe walls, using a spray device that was pulled along inside the pipe. However, spraying has often not been sufficiently effective, because the sprayed material, even if it foams somewhat, has not clung sufficiently to the roots which are to be treated. Since the treatment requires exposing the roots to the phytotoxic material for a period of time, material which simply drips off the roots is often ineffective.
The selection of foam coating or of foam filling may be summarized as follows:
Lines that should be foam-coated:
1. Large lines that are not filled with roots, but have roots around the perimeter of the pipe's interior.
2. Lines that have deep, swift flows that would cause heavy erosion of the foam if the pipe was foam-filled. (Since the nozzle must be above the surface of the flow, the flows could not be deeper than half the pipe. Generally there is a low capacity time of day when flows are lessened; treatments should be reserved for this time of day or night.)
3. Storm sewer lines in which foam-filling the
3. Storm sewer lines would result in the herbicide reaching the outfall in unacceptable amounts.
Lines that should be foam-filled are all other lines that have root intrusions. Roots rated as light to heavy intrusions will soon grow. Early treatment of roots is the best possible Infiltration/Inflow prevention because it stops the roots from cracking the pipe.
Lines that would best be both foam-filled and foam-coated:
1. Large lines that have intermittent massive intrusions of roots can be foam-filled for large intrusions but foam-coated for peripheral intrusions.
2. Storm sewer lines that have massive intrusions of roots can be foam-coated within proper distances from the outfall, but foam-filled where intrusion require it and are far enough from the outfall.
3. Smaller lines which the city feels don't need to be completely treated. Many times a city will try spot-treating lines which have large and very small intrusions intermittently. Although it is not a good policy to ignore the small intrusion, as above, many cities do so. Foam-coating the relatively root-free part of pipes to be spot-treated can be a way to assure some measure of control.
It has been found that generally the application of foam is more effective, especially when it is a clinging foam. The foam-coating method enables economical treatment of large lines. Lines in storm sewers that have outfalls into fresh water sources further profit by this treatment because it allows the lines to be treated without exceeding the amounts of herbicide that can be used in this situation without causing pollution of the watershed. Lines that have a fast, high flow rate would erode the foam too badly to allow the filling of the line. Foam-coating the line in that case is done because it is possible and practical to do so.
Heretofore, the application of foam has usually been done through a conduit that was passed through a plug into a chosen area. The foam generally was relatively slow acting, so that in the past, or stated above, the pipe had to be filled with the foam. Filling the pipe with foam wasted less material than filling the pipe with a liquid solution (because the foam contains twenty times as much air as it does liquid). Excess waste of the foam is not an issue in filling reasonably small lines. But waste can occur when the line is sprayed and the chemical runs away, or foamed when too large to form a plug with the foam itself, because the foam floats on the water below for the most part and erodes or floats away before the line can be filled. The ejected foam, therefore, is ideally sprayed above the liquid flow, because the flow in that case would certainly carry away any foam falling in the water in the foam-coating process. Even in filling the line, because the foam floats, the issue of roots not growing below the surface of the water, except when "horse tails" fill the line, is still not a waste of foam. The foam will catch on horse tails and kill what is above the water. What is in the flow is generally slowly dying and being rotted away by the septic, anaerobic condition of the flow. But the horse tail roots generally enter the line above the flow and the feeder root is killed by the herbicide. The volume of large lines can also result in waste if filled when the roots were only peripheral.